How can I use a trained CNN to predict a new image label?
As the question has been left unanswered, for future readers of the questions:

The documentation you link gives the answer to your question.

Given the fact you have a pre-trained model as you say:

YPred = classify(netTransfer,testDigitData);


where netTansfer is the pre-trained model and testDigitData is your test image that you want to predict the label.

Your image(s) need to be loaded following the instruction also given in the documentation:

digitDatasetPath = fullfile(matlabroot,'toolbox','nnet','nndemos', ...
    'nndatasets','DigitDataset');

digitData = imageDatastore(digitDatasetPath, ...
    'IncludeSubfolders',true,'LabelSource','foldernames');


(digitData will become testDigitData if the images that you load are for testing purpose only, which seems to be your case)